A green tire reinforced with carbon black is provided with a sidewall composition which upon being cured presents a glossy or shiny black surface. The terms xe2x80x9cglossyxe2x80x9d and xe2x80x9cshinyxe2x80x9d are equivalent terms used interchangeably herein, each being measured by reflectivity in a meter designed for the purpose, such as an Erichsen Pico Glossmeter Model 500. Known sidewall compositions comprising an unsaturated rubber, whether predominantly synthetic or natural (1,4-cis polyisoprene) rubber, suffer from weathering due to exposure to atmospheric conditions, including exposure to UV (ultraviolet) light, ozone and high humidity. Such exposure results in fatigue cracking due to continual flexing of the tire sidewall under operating conditions. It is desirable to maintain a cosmetically pleasing appearance of the sidewall without sacrificing the performance characteristics of the sidewall compound.
Tire appearance is a major factor in influencing a customer""s perception of tire quality and results in purchase of the tire by the customer. Pneumatic rubber tires mounted on wheel rims of a vehicle are visually appraised, typically, only in an elevational view which presents the tires"" sidewalls. In the competition for visual attention, the wheel rim easily wins. However, granted the arresting design of a state-of-the-art wheel rim, the drab, dull grey-black appearance of the sidewall denigrates the visual impact of the wheel; it becomes a detrimental distraction and a problem to be coped with, first, when the tire is sold in a salesroom, and then, by the customer who purchased the tire.
A typical mold which is used to cure tires continuously to produce approximately 100 tires during a 24 hr period, is refurbished, that is cleaned and refinished to remove fouling, after about 1500 cures. The period over which tires are produced without having to clean the mold""s sidewall surfaces is referred to as a xe2x80x9cclean runxe2x80x9d. The xe2x80x9cpolishxe2x80x9d or xe2x80x9cfinishxe2x80x9d at the beginning of a clean run, and the ability of the mold""s surface to combat fouling determines, to a large extent, the length of a clean run. To at least the same, if not a greater extent, the length of a clean run is determined by the chemical interaction of ingredients in the sidewall compound. An extension of the clean run of only 10% is economically significant and a greater extension is of major significance.
An uncured sidewall surface of a typical green passenger tire has a gloss no higher than 10; a typical truck tire has an even lower gloss. Over a typical number of curing cycles (xe2x80x9ccuresxe2x80x9d) in a mold, sidewalls have an average gloss lower than 10, typically in the range from about 1 to 6. If refurbished to minimize fouling, a hard smooth chromed surface on a sidewall mold produces a gloss in the range from about 5-15, with a polished surface giving a higher gloss up to about 50 with a 8-microfinish, but upon exposure outdoors over a period of one month, the gloss falls to 5 or less.
To date, one generally copes with the problem by coating the sidewall with a thin protective film, whether liquid or solid, of a glossy compound, e.g. a silicone-containing fluid sprayed or wiped onto a dry and clean sidewall; or a film of polymer such as a waterborne polyurethane or polyepoxide as disclosed in U.S. Pat. No. 6,093,271 to Majumdar, and in WO 01/94453 A1 to Cottin et al.; or by coating with a polyepoxide. Because all sidewalls are subjected to sunlight and oxidation, exposure to ultraviolet light, ozone and high humidity robs black sidewalls of such gloss as they may have had when freshly cured. Continuous distortion under normal operating conditions (of tires), causes high stress and results in fatigue cracking. Abrasion against curbs on paved roads and gravel and stones in unpaved roads, greatly shortens the period of its effectiveness of any coating. Moreover, sidewalls of tires operated on dry pavement quickly acquire a coating of road grime; when run on wet roads, as they are expected to be, they are also coated with mud, in either case requiring repetitive cleaning. Such cleaning damages any thin protective film. Hard experience teaches that the gloss attributable to coatings, to date, is short-lived, lasting less than a month in an outdoor environment. Nevertheless, assuming a coating is highly effective for the purpose, applying the coating is an additional step which the invention disclosed herein now makes unnecessary.
Still another approach to providing a cosmetically enhanced sidewall is to dress up a black sidewall with a band of decorative white rubber, its color due to being heavily reinforced with a light-colored inorganic pigment such as titanium dioxide or calcium carbonate. In addition, or as an alternative, white or palc rubber appliques, whether logos or trade names, are applied to the sidewall to advertise a car owner""s choice of a manufacturer. However, despite the additional expense, the impact of such cosmetic improvements is also relatively short-lived because colored rubber sidewalls and appliques, whether light-colored or dark, are difficult to keep clean, particularly if white, not only because of road grime and mud unavoidably applied exteriorly from the environment, but also because of well-known staining due to conventionally used ingredients of a black sidewall compound, which ingredients include processing oils, antiozonants, antioxidants, zinc oxide and other ingredients in a typical recipe.
Numerous prior art references are directed to sidewall compositions which improve the performance of the sidewall with little or no regard as to what the reflectivity of the sidewall might be, and no suggestion that the glossy or shiny visual appearance of the sidewall, one way or another, might be affected in any way. For example, U.S. Pat. No. 5,066,721 to Hamada et al discloses a silane compound-modified rubbery polymer (xe2x80x9csilanated rubberxe2x80x9d) which may be blended in an amount not less than 10% by weight, with a conventional rubber compound to allow the blend to incorporate from 5 to 90 parts of silica per 100 parts of reinforcement, the remaining reinforcement being carbon black. The effectiveness of the silica is attributed to a silane coupling agent and the silanated rubber in the blend. In a typical recipe (see col 11, lines 33-45) 100 parts by weight of a silane compound-modified rubber (see col 11, line 20) contains 40 phr of silica and 1.5 phr of trioethanolamine. Though the function of the triethanolamine is unstated, it appears to be as a condensation catalyst or an accelerator for the silane coupling agent, and as such it is unclear whether the triethanolamine survives in the cured compound. Such a high level of a tertiary alkanolamine (xe2x80x9ct-alkanolaminexe2x80x9d) such as triethanolamine deleteriously affects the physical properties, particularly the tear strength, of a typical black sidewall compound which contains at least 30 phr of carbon black. A conventional black sidewall compound blended with at least 10 phr of silanated rubber is distinct from a typical black sidewall compound in which, when cured, the t-alkanolamine is reacted with one or more of the ingredients in the sidewall recipe leaving no evidence of the t-alkanolamine originally present. Moreover, a preferred black sidewall compound contains less than 10 parts of silanated rubber per 100 parts, preferably, essentially no silanated rubber.
Several other references refer to shiny sidewalls on cured tires but fail to state what criterion was used to determine that the sidewall was xe2x80x9cshinyxe2x80x9d or whether the shine lasts for any substantial length of time after the tire is removed from the mold, or whether the shine, whatever its original level, can be maintained after exposure to sunlight, and/or to multiple washings with soap and water.
A shiny sidewall is most readily provided by using a highly polished mold surface in contact with the sidewall. To produce a significant number of tires in such a mold, its polished surface must resist fouling. Efforts to do this have produced fouling-resistant coatings on the polished mold surfaces. This invention is concerned with an ingredient which concurrently modifies the sidewall composition and minimizes fouling the surface of the mold.
The Multi-faceted Problem:
It is well known that a highly polished mold surface in contact with a cured sidewall of a black tire results in a shiny sidewall, assuming the use of a release agent which is readily cleaned off the cured surface of the sidewall. The better the microfinish of the surface of the mold, the higher the xe2x80x9cglossxe2x80x9d or xe2x80x9cshinexe2x80x9d. But the gloss lasts less than a month after a cured tire is removed from the curing mold, having been reduced to a dull black by the time the tire is removed from a dealer""s inventory and sold to a customer, typically a longer period under the best of conditions. Restoring the shine by the application of shine-generating fluids does not provide a long-term xe2x80x9cfixxe2x80x9d because such a fix typically lasts a week or less, depending upon the exposure of the tire to sun, rain and other xe2x80x9croad elementsxe2x80x9d.
If the tire is cured in a mold in which the sidewall surfaces are microfinished to mirror-like quality, a freshly cured tire, immediately transported to a salesroom, retains too high a gloss, that is, a glossmeter reading higher than 50. Such a high gloss is deemed objectionable by a typical customer as the gloss, relative to a preferred gloss range, lacks xe2x80x9cjet blacknessxe2x80x9d and therefore appears xe2x80x9cunusualxe2x80x9d. The goal therefore, is to provide a composition for black sidewalls, irrespective of the ingredients of typically carbon black-reinforced rubber compounds for sidewalls, in which black sidewalls the gloss is modulated in a desired range of from about 10 to 40 by the result of a chemical reaction in a curable compound rather than a coating on the cured compound. With a combination of ingredients in the curable compound, sidewalls are to be produced having gloss in the desired range; moreover, such a gloss is to last several weeks even when exposed to sunlight; and when muddied, the gloss can be substantially restored by washing and scrubbing with soap and water.
Addressing the Problem:
After having recognized that a shiny sidewall on a tire emerging from a smooth-surfaced curing mold deteriorates mainly due to chemical interaction of ingredients in the cured sidewall, it was determined that zinc stearate, zinc sulfide (byproducts of reactions during vulcanization), antiozonants, antioxidants and processing oils are mainly responsible for such deterioration. In particular, commonly used antiozonants such as paraphenylenediamine, used to provide protection against attack by ozone under both static and dynamic conditions, and fatty acids used to provide a sidewall compound with better processing characteristics, are particularly aggressive in their attack on the once-shiny sidewall black surface which is stained or otherwise reduced to a dull grey-black color. The term xe2x80x9cstainxe2x80x9d or xe2x80x9cstainingxe2x80x9d is herein used to describe the proclivity of a material to diffuse through a polymeric substrate and discolor its surface, whatever its original color. In black sidewalls, diffusion of paraphenylenediamine to the surface results in a bluish grey, then a dull brown surface which is aesthetically objectionable if one expects a glossy jet black smooth appearance, such as is reasonably expected in a new black sidewall tire. Moreover, the dull grey or brown surface discoloration is difficult to remove by routine cleaning with soap and water.
Modification of a black rubber sidewall compound by the addition of a combination of less than 10 phr of a xe2x80x9ctackifierxe2x80x9d and less than 1 phr of a t-(lower)-alkanolamine (2,2,2,-nitrilotrisalkanol) generates, upon curing, a reaction product which, at the surface of the sidewall, produces a gloss in the range from about 10 to 40 lasting at least one month without diminishing the mechanical properties of the sidewall and the operating performance of the tire relative to that of a tire without the added combination. Multiple washing of the sidewalls during the period fails to diminish the gloss substantially.
A curable black rubber sidewall compound essentially free of a silanated rubber, is compounded with a combination of from about 0.1-10 phr of a rosin acid type tackifier, and from 0.05 to less than 1 phr (part per 100 parts of rubber) of a t-C1-C5-alkanolamine, without the addition of a fatty acid. The compound exhibits desirable ozone resistance, heat dissipation and fatigue crack propagation resistance despite containing in the range from about 30 phr to 75 phr of reinforcing filler, the major portion by weight of which is carbon black. The cosmetically pleasing appearance of the sidewall is maintained when the tire is cured in a metal mold which delivers a clean run (the surface of the mold fails to get fouled enough to require refurbishing) which run is at least 20% longer than a clean run with the same black rubber compound without the tackifier and t-alkanolamine.
A curable tire sidewall composition comprises from about 30 phr to 75 phr of reinforcing filler, more than 90% of it being carbon black, and either (i) a blend of highly unsaturated rubbers, e.g. cis-polybutadiene and natural rubber, or (ii) a blend of from about 40 to 60 phr of a highly unsaturated rubber with correspondingly from about 60 phr to 40 phr of one of lesser unsaturation, in either of which blends (i) or (ii) the only fatty acid(s) is that provided by natural rubber present in a minor amount by weight relative to the other rubbers; and, process- and performance-enhancing ingredients including from 0.1-5 phr of a rosin acid type tackifier, and from about 0.05 phr to 0.75 phr (part per hundred parts by weight of rubber) of the t-alkanolarnine, optionally with from 0.1 to less than 1 phr of a polyalkylene glycol having a molecular weight in the range from about 200 to 1,000, preferably 300 to 600.
A curable sidewall compound is compounded by blending a highly unsaturated rubbery polymer with an elastomer having lesser unsaturation than the highly unsaturated rubbery polymer, process- and performance-enhancing ingredients including from 0.1-3 phr of a rosin acid type tackifier, and from about 0.05 phr to 0.75 phr of a t-alkanolamine, the alkanol having from 1 to 5 carbon atoms.
An ATR-FTIR spectrum indicates that the t-alkanolamine in the uncured rubber is not present in the cured sidewall compound as a result of having reacted at curing temperature and pressure. The overall effect of the reaction product is that it masks the dulling effect of both aromatic processing oils and zinc oxide, both essential ingredients in the recipe for a sidewall.